Add support for other multi-threading APIs

[krzikalla]

Support the benchmarking of code, which relies on other multi-threading APIs, e.g. OpenMP.

[krzikalla]

This is a follow-up to PR https://github.com/google/benchmark/pull/1027 The approach is basically the same, it now addresses most of the comments made there.

[LebedevRI]

I guess i will need to review this. It's unfortunate that so much time has passed since the previous attempt, i basically do not recall what specific issues i had there.

[krzikalla]

I guess i will need to review this. It's unfortunate that so much time has passed since the previous attempt, i basically do not recall what specific issues i had there.

Any chance to get a review on this soon? Do I have to do something before you start? The issues I still have in the CI runners don't seem to be related to my additions.